So far, as Mach-Zehnder optical modulators having a waveguide structure, there have been known a type using a Z-cut substrate obtained by cutting a crystal with an electro-optical effect, such as LiNbO3 [lithium niobate (LN)] crystal or LiTaO2 [lithium tantalate (LT)] crystal, in a Z-axis direction of the crystal orientation and a type using an X-cut substrate obtained by cutting it in an X-axis direction of the crystal orientation.
FIG. 11A is a plan view illustratively showing a structure of an optical modulator (Z-cut modulator) using a Z-cut substrate, FIG. 11B is a cross-sectional view taken along A-A in FIG. 11A, FIG. 12A is a plan view illustratively showing a structure of another optical modulator (X-cut modulator) using an X-cut substrate and FIG. 12B is a cross-sectional view taken along A-A in FIG. 12A.
First of all, as shown in FIG. 11A, on a Z-cut substrate 100, the Z-cut modulator has, for example, an incidence waveguide 101, a incidence side Y-branch wave guide 102, parallel waveguides 103A and 103B, an outgoing side Y-branch waveguide 104 and an outgoing waveguide 105, with a signal electrode 106 and earth electrodes 107, 108 being patterned thereon.
For the formation of such a Z-cut modulator, waveguides 101, 102, 103A, 103B, 104 and 105 are produced by forming a metallic film on a portion of a crystal substrate and making thermal diffusion thereon or by patterning a metallic film and then making proton exchange or the like in benzoic acid, thereafter placing electrodes in the vicinity of the parallel waveguides 103A and 103B.
However, since the Z-cut modulator utilizes a variation of refractive index stemming from an electric field in the Z direction, as shown in FIG. 11B, portions of the electrodes 106 and 108 are located right above the parallel waveguides 103A and 103B, respectively.
On the other hand, as shown in FIG. 12A, on an X-cut substrate 100′, the X-cut modulator has, for example, an incidence waveguide 101, an incidence side Y-branch waveguide (branching unit) 102, parallel waveguides 103A and 103B, an outgoing side Y-branch waveguide (combining unit) 104 and an outgoing waveguide 105, with a signal electrode 106 and earth electrodes 107, 108 being patterned thereon. Because of the utilization of a variation of refractive index stemming from an electric field in the Z direction, as shown in FIG. 12B, the parallel waveguides 103A and 103B are positioned between the signal electrode 106 and the earth electrode 107 and between the signal electrode 106 and the earth electrode 108, respectively.
For preventing light propagating in waveguides from being absorbed by the earth electrodes 107 and 108, as shown in FIGS. 11B and 12B, a dielectric layer (buffer layer) 109 is usually placed between the substrate 100 (100′) and the signal electrode 106, the earth electrodes 107, 108. As the buffer layer 109, for example, there is used SiO2 having a thickness of 0.2 to 1.0 μm.
In each of the Z-cut modulator and the X-cut modulator, configured as mentioned above, when light is incident on the incidence waveguide 101, this incident light is led from the incidence waveguide 101 to the incidence side Y-branch waveguide 102 where it is divided into two in the same power ratio so as to propagate in the parallel waveguides 103A and 103B.
At this time, in a case in which a microwave electric signal [drive voltage; half-wavelength voltage Vπ] is applied to the signal electrode 106, as shown in FIG. 11B or 12B, the refractive indexes of the two parallel waveguides 103A and 103B respectively vary as +Δna and −Δnb due to the electric field stemming from this microwave electric signal and the phase difference between the parallel waveguides 103A and 103B, so the light combined in the outgoing side Y-branch waveguide 104 becomes intensity modulated light and is outputted from the outgoing waveguide 105.
In this case, it is known that, in the Z-cut modulator, modulated light with a wavelength chirp (quantity) α=−0.7 is obtained because of Δna:Δnb to 5:1 while, in the X-cut modulator, modulated light with a wavelength chirp α=0 is obtained because of Δna:Δnb to 1:1. Accordingly, in many cases, the Z-cut modulator is used for the long-distance optical transmission while the X-cut modulator is for use in the relatively-short-distance optical transmission.
As a conventional technique on an optical modulator, there exists a technique proposed in the following Patent Document 1, and as a conventional technique using an optical modulator, there exists a technique proposed in the following Patent Document 2.
The technique in the Patent Document 1 is for the purpose of, in an optical modulator suitable for use in an optical communication system, suppressing the generation of radiation mode light to improve the performance as a device. Thus, in the optical modulator disclosed in the Patent Document 1, a combination waveguide (3 dB) coupler includes two input waveguides for receiving propagation lights from two linear waveguides, a waveguide coupling unit and two output waveguides, and the input side width of the waveguide coupling unit is set to be larger than the total width of the two input waveguides and the output side width of the waveguide coupling unit is set to be larger than the total width of the two output waveguides. This can suppress the generation of the incidence side and outgoing side radiation mode lights with respect to the waveguide coupling unit, thereby enhancing the performance as a device.
The technique in the Patent Document 2 is for the purpose of providing an optical transmitter having a high proof strength with respect to the group velocity dispersion of an optical fiber, having a low reception sensitivity degradation and less susceptible to the influence of the group velocity dispersion at the enlargement of the network scale. Thus, an optical transmitter is composed of a light source section capable of generating an optical clock pulse synchronized with a signal bit rate in a given duty ratio and variably setting the duty ratio of the optical clock pulse and an encoding section for setting the relative optical phase difference between the optical clock pulses of the adjacent time slots at odd number times of π and encoding an optical clock pulse through the use of an electric signal synchronized with the optical clock pulse. Therefore, since the duty ratio of the optical clock pulse is variable, a high dispersion proof strength and a small reception sensitivity degradation are compatible by setting the duty ratio at an appropriate value. Moreover, since the relative optical phase difference between the optical clock pulses of time slots adjacent to each other is set at the odd number times of π or a value approximate thereto, a high dispersion proof strength is stably maintainable. That is, the configuration of a stable optical transmitter becomes feasible, which enables a suitable construction of a network with a larger scale.
However, in the case of the above-mentioned conventional techniques, there is a need to prepare a dedicated optical modulator (Z-cut modulator or X-cut modulator) having a different wavelength chirp α according to a needed optical transmission distance at the construction of an optical transmission system. In addition, when the optical transmission distance is changed after the system construction, for example, there is a need to replace the optical modulator with one having a desired wavelength chirp α, which leads to low flexibility.
For this reason, so far, for example, as disclosed in the following Non-Patent Document 1, in a 1-input 1-output type optical modulator having a basic structure similar to the optical modulators mentioned above with reference to FIGS. 11A and 12A, there is proposed a technique of applying a direct-current (DC) voltage to an incidence side Y-branch waveguide (branching unit) so that the branch ratio of the incident light in the incidence side Y-branch waveguide becomes variable to make the wavelength chirp quantity variable (see FIG. 13).
However, this technique cannot simply provide output light with a different wavelength chirp quantity.
The present invention has been developed in consideration of these problems, and it is an object of the invention to simply provide output light with a different wavelength chirp quantity. Patent Document 1: Japanese Patent Laid-Open No. 2003-329986 Patent Document 2: Japanese Patent Laid-Open No. 2001-339346 Non-Patent Document 1: AVANEX corporation, “Single-Drive LiNbO3 Mach-Zehnder Modulator With Widely DC Tunable Chirp”, IEEE Photon. Technol. Lett., Vol. 15, pp. 1534-1536 (2003).